1. Field of the Invention
The present invention pertains to apparatus and methods for detecting the extent of the presence of material at a predetermined sensing area. More particularly, the present invention relates to systems for determining the amount of material accumulated at a locale. Apparatus and methods of the present invention find particular application in cases of material storage in containers, such as bins, wherein the amount of material present may be determined by detecting the depth of material in the bin. Appropriate indicators, or warning devices, may be operated ultimately by use of the present invention, and systems for adding material, or removing material, may also be operated in response to the sensing of the amount of material present by use of the instant invention.
2. Description of the Prior Art
Several prior art systems for measuring material levels in bins include the use of mechanical probes which extend into the bin. Such a probe is made to vibrate, and the physical characteristics of the elastic disturbances, which are affected by the quantity of material present about the probe, are analyzed in terms of the quantity of material present.
Other prior art devices rely upon the variation of one or more electrical properties affected due to the presence of material at different levels within the bin. Thus, capacitive reactance of one or more probes may be affected to signal the amount of material present. The operation of an oscillator circuit, including a resonant probe, may be affected to operate a control system in response to the presence of material near the resonant probe.
U.S. Pat. Nos. 3,807,231 and 3,935,970 disclose a measuring system and apparatus wherein the reactance of an antenna probe varies as a function of the level of material in the container adjacent the antenna. The antenna reactance controls the frequency of a transmitter signal, which is then combined with a constant frequency reference signal. The frequency of the resulting difference signal is used to operate a material level indicator and to provide control information for operating automatic systems for controlling the material level.
An amplitude-modulated detector circuit is disclosed in United States patent application Ser. No. 844,040 filed Oct. 20, 1977 and assigned to the Assignee of the present invention. There, a crystal-controlled oscillator generates a constant-amplitude signal which is fed through a load resistor to an antenna probe circuit. Variations in the amplitude of this loaded signal, due to the presence of material to be detected about the antenna probe, are reflected in variations in the value of a d-c voltage level. The variable voltage level is combined with a reference voltage level and the resulting signal is fed to a switching circuit, The switching circuit provides an output signal depending on a comparison between the combined signal and a second reference level.
A detector circuit is disclosed in United States patent application Ser. No. 877,028, filed Feb. 10, 1978 and assigned to the Assignee of the present invention. There, an antenna probe forms an arm of an impedance bridge on which is impressed a constant-frequency, constant-amplitude signal generated by an oscillator. Presence of material at the antenna probe affects the impedance of the bridge and accordingly alters the amplitude of the signal which is then converted to a d-c voltage level. A switching circuit includes a comparator which compares the d-c voltage level with a reference level, and produces an output signal indicative of the presence or absence of material at the antenna probe. The sensitivity of the switching circuit may be adjusted to accomodate variations in the effect different materials to be detected may have on the impedance of the antenna probe. The switching circuit itself is equipped with a light emmitting diode, and functions as a calibration circuit for the detector circuitry.
The advantage of matching the sensitivities of such detector circuits to the material to be detected is recognized in the art. The effect on the impedance of detector antenna probes is directly dependent on the electromagnetic character of the material being detected. Regardless of the magnitude of the antenna load impedance variation effected by the presence of material in the storage area, the detector circuit output signal must exhibit a gross change to reflect the level of material in the storage area. Consequently, where the material being stored exhibits but a slight effect on the antenna load impedance, the sensitivity of the detector circuit must be sufficiently acute to respond. However, in the case of storage materials producing greater effects on the antenna load impedance, the sensitivity of the detector may be reduced. In any case, if the sensitivity of the detector circuit is too low in relation to the material being detected, the detector circuit may not respond to the presence of such material surrounding the antenna probe. On the other hand if the sensitivity of the detector circuit is too high, false readings of the presence of material at the antenna probe may result due to stray electrical noise, the flow of material past the antenna probe as the storage area is being filled, or possibly even mechanical vibrations.
The aforementioned Ser. No. 877,028 application includes a provision for readily adjusting the sensitivity of an amplitude-modulated discrete point level detector. Currently, it is known to construct a frequency-modulated continuous level detector with the sensitivity of the circuitry matching anticipated material to be detected by appropriately constructing hand-wound coils for use in the oscillator circuits. However, such a technique does not readily permit alteration of the circuitry sensitivity where the detector is to be used with a variety of storage materials.